1. Field of Invention
The present invention pertains to the field of magnetrons. More particularly, this invention relates controlling target power in a magnetron.
2. Art Background
A magnetron is a device that is commonly used for depositing a film onto a surface. The process of depositing a film onto a surface using a magnetron is commonly referred to as sputtering. For example, a magnetron may be used to sputter an aluminum-nitride film onto a silicon wafer. An aluminum-nitride film on a silicon wafer may be a step in the manufacture of piezoelectric acoustic resonator filters including film bulk acoustic resonator (FBAR) filters.
A typical magnetron includes a chamber and one or more targets that are bombarded with ions. The ion bombardment of the targets usually causes erosion of the target material. Typically, eroded material from the targets is deposited as a film onto a wafer contained in the chamber. The target material is usually selected to yield a particular substance to be deposited on the wafer. To deposit an aluminum-nitride film onto a silicon wafer, for example, the target material is typically aluminum.
The sputtering characteristics of the targets in a magnetron typically change throughout the life of the targets. For example, the cumulative effect of material erosion from a target gradually changes the shape of the surface of the target throughout its life. A change in the shape of a target surface usually changes the angle at which material is eroded from the target. A change in the angle of material erosion from a target usually changes the rate of material deposition from that target onto a wafer. In addition, the changes to rates of material deposition usually varies among the targets in a magnetron.
Such variation in the sputtering characteristics among targets in a magnetron usually degrades the uniformity in thicknesses of films deposited on wafers as the targets age. Unfortunately, such non uniformity in film thickness usually creates undesirable film characteristics. In an FBAR filter, for example, the lower the uniformity in the aluminum-nitride film thickness the lower the yield of the resulting filters.
A magnetron is disclosed with mechanisms for smoothly and continuously adjusting a DC power applied to its targets to compensate for the changes in the sputtering characteristics of the targets that occur with target aging. A magnetron according to the present teachings includes a set of concentric targets for sputtering a film onto a wafer in response to an AC power and a DC power applied to the targets and a power controller that adjusts the DC power. The adjustments to the DC power enable the magnetron to maintain uniformity in the thicknesses of films formed with the magnetron throughout the life of its targets.
Other features and advantages of the present invention will be apparent from the detailed description that follows.